hgdemandimport/demandimportpy3.py
author Augie Fackler <augie@google.com>
Wed, 12 Jun 2019 16:08:21 -0400
changeset 42474 adb636392b3f
parent 37843 670eb4fa1b86
child 43076 2372284d9457
permissions -rw-r--r--
demandimport: add tracing coverage for Python 3 This makes things feel a little less mysterious when modules are being imported. Differential Revision: https://phab.mercurial-scm.org/D6523
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# demandimportpy3 - global demand-loading of modules for Mercurial
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#
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# Copyright 2017 Facebook Inc.
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#
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# This software may be used and distributed according to the terms of the
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# GNU General Public License version 2 or any later version.
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"""Lazy loading for Python 3.6 and above.
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This uses the new importlib finder/loader functionality available in Python 3.5
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and up. The code reuses most of the mechanics implemented inside importlib.util,
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but with a few additions:
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* Allow excluding certain modules from lazy imports.
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* Expose an interface that's substantially the same as demandimport for
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  Python 2.
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This also has some limitations compared to the Python 2 implementation:
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* Much of the logic is per-package, not per-module, so any packages loaded
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  before demandimport is enabled will not be lazily imported in the future. In
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  practice, we only expect builtins to be loaded before demandimport is
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  enabled.
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"""
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# This line is unnecessary, but it satisfies test-check-py3-compat.t.
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from __future__ import absolute_import
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import contextlib
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import importlib.abc
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import importlib.machinery
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import importlib.util
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import sys
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from . import tracing
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_deactivated = False
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class _lazyloaderex(importlib.util.LazyLoader):
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    """This is a LazyLoader except it also follows the _deactivated global and
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    the ignore list.
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    """
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    def exec_module(self, module):
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        """Make the module load lazily."""
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        with tracing.log('demandimport %s', module):
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            if _deactivated or module.__name__ in ignores:
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                self.loader.exec_module(module)
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            else:
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                super().exec_module(module)
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# This is 3.6+ because with Python 3.5 it isn't possible to lazily load
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# extensions. See the discussion in https://bugs.python.org/issue26186 for more.
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_extensions_loader = _lazyloaderex.factory(
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    importlib.machinery.ExtensionFileLoader)
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_bytecode_loader = _lazyloaderex.factory(
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    importlib.machinery.SourcelessFileLoader)
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_source_loader = _lazyloaderex.factory(importlib.machinery.SourceFileLoader)
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def _makefinder(path):
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    return importlib.machinery.FileFinder(
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        path,
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        # This is the order in which loaders are passed in in core Python.
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        (_extensions_loader, importlib.machinery.EXTENSION_SUFFIXES),
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        (_source_loader, importlib.machinery.SOURCE_SUFFIXES),
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        (_bytecode_loader, importlib.machinery.BYTECODE_SUFFIXES),
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    )
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ignores = set()
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def init(ignoreset):
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    global ignores
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    ignores = ignoreset
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def isenabled():
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    return _makefinder in sys.path_hooks and not _deactivated
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def disable():
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    try:
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        while True:
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            sys.path_hooks.remove(_makefinder)
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    except ValueError:
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        pass
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def enable():
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    sys.path_hooks.insert(0, _makefinder)
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@contextlib.contextmanager
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def deactivated():
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    # This implementation is a bit different from Python 2's. Python 3
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    # maintains a per-package finder cache in sys.path_importer_cache (see
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    # PEP 302). This means that we can't just call disable + enable.
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    # If we do that, in situations like:
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    #
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    #   demandimport.enable()
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    #   ...
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    #   from foo.bar import mod1
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    #   with demandimport.deactivated():
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    #       from foo.bar import mod2
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    #
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    # mod2 will be imported lazily. (The converse also holds -- whatever finder
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    # first gets cached will be used.)
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    #
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    # Instead, have a global flag the LazyLoader can use.
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    global _deactivated
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    demandenabled = isenabled()
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    if demandenabled:
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        _deactivated = True
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    try:
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        yield
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    finally:
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        if demandenabled:
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            _deactivated = False